Implantable pacemakers improve cardiac function by improving electrical activation of the heart. It has been demonstrated that the degree of improvement in cardiac function may depend on the locations of pacing sites, vector configuration, and the programmable parameters that control the timing of pacing pulse. Cardiac function is quantified by, for example, cardiac output and filling pressures. In general, the use of implantable pacemakers to improve cardiac function is referred to as cardiac resynchronization therapy (CRT) and implemented using a biventricular cardiac pacemaker that can pace both the right and left ventricle. The pacemaker is able to resynchronize a heart, whose right and left ventricles do not contract in synchrony, by pacing both the right and left ventricles. Biventricular pacemakers have at least two leads, one in the right ventricle to stimulate the septum, and the other inserted through the coronary sinus to pace the lateral wall of the left ventricle. An additional lead in the right atrium can facilitate synchrony with atrial contraction.
Programmable biventricular pacemakers enable optimization of treatment for a particular patient. The various time delays between pacemaker timing pulses can be adjusted and set for each patient. The optimization procedure generally requires a physician or nurse to set delays between various timing pulses. The purpose of the optimization is to coordinate contraction of the various chambers in response to the various cardiac pulses to improve overall efficiency and function of the heart. Adjustment of atrioventricular (AV) pacing delays allows optimization of the time interval between paced or intrinsic atrial contraction and the paced ventricular beat for best cardiac efficiency. It is generally believed that both ventricles should contract simultaneously for optimum cardiac performance; however, interventricular (VV) pacing delay is often also required to obtain contraction. While optimal pacing sites and pacing parameters can provide the largest improvement in cardiac function, the parameters are patient-specific and may change over time.
Most commonly, optimization of pacing parameters is done during follow up visits using echocardiography. Parameters that determine timing, such as AV delay and VV delay are programmed to different values, and at each setting a particular echocardiographic index is measured. In many commercial pacemakers, a wireless communication system allows for this external programming. Generally, the pacemaker includes a short range telemetry module that communicates with an external device to facilitate communications with the implanted pacemaker.
Echocardiographic optimization is subject to a number of problems. For example, it is time and resource consuming. In addition, echocardiographic optimization may have limited reproducibility and greater inter- and intra-operator variability. Further, echocardiography is ordinarily used to optimize settings only when a patient is at rest. For these reasons, some clinicians may not routinely perform optimization. Moreover, using echocardiographic optimization, there still may be numerous patients who are non-responders to the therapy.